I'm almost positive it doesn't matter. For example when they're reviewing a fumble. It's if the ball came out before or after his first knee touched the ground thus being "down." It's the same thing in the end zone.His other knee and feet don't matter because he had secured the ball at the point that his knee touched the surface in bounds.

When it's in regards to a catch.. in order for it to be a completion.. the WR has to first have both feet be inbounds and has to have full control of the ball throughout the full motion of the catch. so in Ballards case.. he caught the ball had full control over it and made contact inbounds with the one leg but did not get both knees or feet inbounds therefore it was ruled an incompletion.